1. Field of the Invention
The invention relates generally to pulsed radar systems and more specifically to multi-stage RF cross-field amplifier pulse modulators.
2. Description of the Prior Art
In the radar field, two fundamental pulse transmitter configurations are extant, one involves a pulsed microwave oscillator RF generator, such as the familiar magnetron, and the other involves a low power frequency programmed generator and a plurality of microwave power amplifiers successively boosting the RF power to the desired output level. Each stage in the latter configuration must be separately and synchronously pulse modulated and it is this arrangement to which the combination according to the invention relates.
The common prior art of power pulse modulators for microwave RF generators includes the so-called line-type modulators and hard tube modulators. Line-type modulators are simple and usually involve the charging and switched discharge of an artificial transmission line to provide a high power modulation pulse at high peak voltage. Such line-type modulators are typically used to drive the magnetron oscillator aforementioned.
This line-type modulator is limited in its capability and flexibility in that the modulation pulse width is determined by the line itself and is therefore fixed. Moreover, parasitic circuit elements and responses are difficult to control and can impact pulse fidelity if the line (pulse forming network) is not located in close proximity to the pulse transformer through which the pulses are applied to the corresponding RF generator.
Still further, a line-type modulator is basically a voltage source, as that term is understood in this art, whereas, for CFA transmitters, a current source produces much better CFA performance.
Some type of pulse amplitude regulation must be provided for the output pulses of a line-type pulse modulation for pulse-to-pulse consistency of frequency generated by a pulsed mangetron, for example. Such consistency is necessary to achieve stabilities on the order of 40 to 50 decibels for associated MIT (moving target indicator) circuits. Although the line-type modulator is relatively power efficient per se, overall power efficiency of a line-type pulse modulator system considering line charging and amplitude regulation, may be as low as fifty percent.
The so-called hard tube modulator is initially simple and straightforward in that it utilizes a large transmitting type vacuum tube as a switch controlled by grid pulsing circuits. Such an arrangement may be characterized as a "brute force" approach which simply switches the power source to the RF generator as a load during the pulse time as determined by the corresponding grid pulsing.
The large vacuum tube required by a hard tube modulator consumes substantial filament (heater) power and usually requires forced air cooling. The grid-pulser must float below ground at the cathode potential of the switch tube, and a screen supply must be provided if the large vacuum tube is a tetrode. Further design complication results if a feedback circuit is provided to raise the dynamic output inpedance of the modulator.
Although the hard tube modulator is pulse width and repetition frequency agile, its efficiency and reliability are less than optimum and fail-soft features cannot be incorporated. The high voltage power supply required is also a significant disadvantage.
A radar system text of use for overall background in this art is the book "Radar Handbook" by Merrill I. Skolnik (McGraw Hill 1970).
The manner in which the invention addresses the modulator requirement for a multi-stage CFA transmitter will be understood as this specification proceeds.